The present invention relates generally to coin sensors and more specifically to inductive coin sensors for identifying a variety of coins.
Coin operated machines must have the capability of determining the validity of an inserted coin or token as well as its value. A typical coin sensor utilizes inductive electromagnetic fields created by energized sensor windings to sense coins. A coin inserted into a coin slot of the coin sensor travels through an electromagnetic field causing field variations as it travels. Characteristics of observed frequency and amplitude changes of the oscillating electrical signals caused by the field variations are compared with stored expected values for a variety of coins. If the characteristics of the inserted coin are not within the predetermined recognizable limits of the stored expected values, then the coin is not accepted and returned to the patron. For example, coins are often worn or otherwise damaged due to use which results in unrecognizable characteristics.
The prior art method of coin detection often leads to frustration for the patron since, more often than not, worn and/or damaged coins are rejected because the coins display characteristics outside of the acceptable limits. The determined patron struggles with a particular sensor, often without success, by feeding the same coin into the sensor over and over again. In applications such as vending machines for transit fares, an improperly-functioning vending machine not only damages the business reputation of the responsible transit authority, but may cause a patron to miss a transit connection.
The prior art coin sensors also present other inconveniences and shortcomings. The arrangement of the windings of the prior art often necessitate the feeding of a single coin through the sensor area before another coin can be accepted for verification. Another problem faced by the prior art coin sensors is the necessity for calibration due to the aging of the coin detection device and temperature and humidity variations. Proposed calibration techniques include storing a range of values for each coin to compensate for the calibration variations. Another technique employs a reference oscillator to generate correcting signals for use by the sensor circuitry. Still another technique employs introducing a calibration signal into the windings to produce a response that is then used to calibrate the responses due to actual coins. These solutions add complexity to the sensor circuit and cannot compensate for all variations that may occur during operation of the coin sensor.
A number of coin sensor configurations have been proposed to overcome the deficiencies of the basic inductive sensor including the use of a number of fields with different frequencies to measure more than one characteristic of the coin. Another configuration utilizes the change in amplitude of a field over time to identify a particular coin. Although a number of comparison factors may increase accuracy, the coin sensor maintains the problems of inaccuracies due to calibration and the positioning of the coin as it passes through the sensor. Thus, the need remains for an efficient and accurate coin sensor.
It is an advantage of the present invention to provide a component-efficient sensor that minimizes the need for complex circuitry.
It is another advantage to provide a sensor that minimizes the detrimental effects of the lateral, longitudinal, and transverse positioning of a coin through a sensor.
Yet another advantage of the present invention is to provide a coin sensor that is not affected by changes in oscillator frequency due to long term drift caused by component aging and environmental changes.
Still another advantage is to provide a coin sensor that does not require control of coin velocity nor control of the lateral, longitudinal or transverse position of the coin.
It is a further advantage to provide a coin sensor with a plurality of oscillators that are time-division multiplexed to prevent interaction between the oscillators in order to obtain accurate frequency profiles, and to minimize circuit complexity and product size.
Further advantages and objects of the present invention will be apparent from the following description of the invention.
In an exemplary embodiment, a coin is introduced into a coin slot of a coin sensor and travels through the magnetic fields of three sets of windings before exiting the coin sensor. The magnetic fields are produced by four inductive/capacitive (LC) oscillators. A first set of windings, corresponding to a first pair of oscillators, is split into two halves, one half in an upper portion of the coin sensor, i.e., above the coin slot, and the other half in a lower portion of the coin sensor, i.e., below the coin slot. The first set of windings is oriented to generate magnetic flux lines perpendicular to the faces of the coin in the coin slot. As the coin travels through the magnetic field of the first set of windings, the inductance of the oscillator drops, causing a rise in the frequencies F1A and F1B of the first pair of oscillators, wherein the rise in frequencies F1A and F1B is due primarily to the facial area of the coin.
The coin sensor of the exemplary embodiment further includes a second set of windings corresponding to a third oscillator. The second set of windings surrounds the coin slot in such a way as to generate magnetic flux lines that are parallel to the faces of the coin as it travels through the coin slot. The presence of the coin in the magnetic field causes a drop in the inductance of the third oscillator, resulting in a rise in frequency F2 of the oscillator. The rise in frequency F2 is due to the cross-sectional area, i.e., the thickness multiplied by the diameter, of that portion of the coin within the field. A third set of windings, driven by a fourth oscillator, is split in two half-coils that are separated longitudinally in the direction of coin motion. The third set of windings produces two magnetic fields that are perpendicular to the faces of the coin as it travels through the coin slot. The two half-coils of the third set of windings are utilized to further distinguish the relative size of an inserted coin. Particular coin sizes may interact more strongly when both half-coils cover portions of the coin, while other coin sizes may interact more strongly when a single half-coil covers a portion of the coin.
The four oscillators of the exemplary embodiment are operated in a time-division multiplex fashion utilizing control lines controlled by a microcontroller. The time-division multiplexing allows all frequencies of the three sets of windings to be measured by a single counter. In addition, time-division multiplexing ensures that the magnetic fields of the windings do not interact, thus providing predictable frequency changes for a particular valid coin type. The frequencies of the four oscillators are counted utilizing a counter that is multiplexed to the outputs of the oscillators in a predetermined sequence and for a pre-determined duration. The microcontroller accumulates samples of each of the frequencies from the counter and stores the results in a memory. The samples of the frequencies are then utilized by the microcontroller to produce three frequency profiles for the inserted coin, wherein the first frequency profile corresponds to the first set of windings, the second frequency profile corresponds to the second set of windings, and the third frequency profile corresponds to the third set of windings.
Specific points of the frequency profiles are extracted to identify the inserted coin. In the exemplary embodiment, a frequency point of each of the three frequency profiles is identified for the coin when it is centered in the coin slot. After compensation for transverse position, these three points are used directly as the signature for the inserted coin, and are sufficient for the identification of most coins. However, the method of the exemplary embodiment may identify other points, such as cross-over points where one frequency profile crosses another, to further define a signature. Once the microcontroller has determined a signature for the inserted coin, it compares the signature to pre-stored signatures for a variety of valid coins and/or tokens. If a match is found for all points of the signature, then the inserted coin is identifiable for further processing, e.g., for acceptance or rejection based upon the particular requirements of the mechanism utilizing the coin sensor.